Toxic oil syndrome was a devastating food-contamination disaster in Spain in 1981, when industrial rapeseed oil denatured with aniline — intended for industrial use, never for food — was illegally refined and sold as cheap olive oil. Tens of thousands of people fell ill with a mysterious illness causing lung damage, muscle pain, and immune dysfunction, and hundreds died. The outbreak, abbreviated TOS, took years to trace to its source and remains one of history’s worst cases of economically motivated food fraud. It transformed Spanish and European food safety law, strengthening traceability, fraud enforcement, and the regulation of industrial oils. Memorials in Madrid honour the victims of the poisoned oil.